19 July 2010

How Much Warming Are We Asking For?

"A New Epoch"

A recent report from the U.S. National Research Council does not mince words.
"Emissions of carbon dioxide from the burning of fossil fuels have ushered in a new epoch where human activities will largely determine the evolution of Earth’s climate. Because carbon dioxide in the atmosphere is long lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe. Therefore, emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millennia." [From the executive summary]
Key points of the report:
  • Because CO2 persists so long in the atmosphere, our emissions today and over the rest of the century will have profound effects on Earth's systems for centuries or even millennia to come. 
  • To achieve atmospheric CO2 stabilization at any given level, emissions will have to be cut by more than 80% from current rates. The longer we wait to make those cuts the higher the level will be when it stabilizes.
  • Global average temperatures can be correlated with atmospheric levels of carbon dioxide, so we can make predictions of future warming from projections of future emissions.
  • A growing body of research allows specific likely changes in Earth's systems to be forecast based on projected average global temperatures.
  • "The report concludes that certain levels of warming associated with carbon dioxide emissions could lock the Earth and many future generations of humans into very large impacts; similarly, some targets could avoid such changes."

Projected climate impacts

The report, "Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia", addresses this situation by providing estimates of the impacts from various levels of global warming and CO2 emissions. The authors hope policymakers will take advantage of this information.
The report estimates changes in precipitation, streamflow, wildfires, crop yields, and sea level rise that can be expected with different degrees of warming. It also estimates the average temperature increases that would be likely if CO2 were stabilized in the atmosphere at various target levels. However, the report does not recommend any particular stabilization target, noting that choosing among different targets is a policy choice rather than strictly a scientific one because of questions of values regarding how much risk or damage to people or to nature might be considered too much. [From the press release]
(The full report is available here You have to enter an email address on a form to download the free PDF. A PDF of executive summary here.)

The report tries to convey a sense of urgency by reminding us that "climate change due to carbon dioxide will persist many centuries". This is because CO2 remains so long in the atmosphere. Even if we were to reduce our emissions immediately the excess CO2 we have already put into the atmosphere will continue to drive changes in the Earth's climate for decades or even centuries to come.

And the authors note that "Depending on emissions rates, carbon dioxide concentrations could double or nearly triple from today’s level by the end of the century, greatly amplifying future human impacts on climate." Translation: if you think the consequences we have already set in motion by past and current emissions are unattractive, just imagine the impact of annual emissions several fold higher continuing for many decades.

The Future

What will those quantities of carbon dioxide mean in terms of actual warming, and what will that warming do to the environment and society? First, the report estimates the warming we are buying with our current and future emissions. This table shows estimated eventual warming at various "stabilization" levels of CO2. (Today's level is about 390 ppm, but of course we are not stabilizing at that level.)
Here are some of the corresponding impacts the report predicts:
  • "For each degree (°C) of global warming:

    • 5-10% changes in precipitation in a number of regions
    • 3-10% increases in heavy rainfall
    • 5-15% yield reductions of a number of crops
    • 5-10% changes in streamflow in many river basins worldwide
    • About 15% and 25% decreases in the extent of annually averaged and September Arctic sea ice, respectively
  • For warming of 2°C to 3°C, summers that are among the warmest recorded or the warmest experienced in people’s lifetimes, would become frequent.
  • For warming levels of 1°C to 2°C, the area burned by wildfire in parts of western North America is expected to increase by 2 to 4 times for each degree (°C) of global warming."

We can fix it, right?

To make warming level off at any particular global temperature, the concentration of carbon dioxide would have to be stabilized at some level. But because human-caused emissions are rising sharply, and because past emissions will linger in the atmosphere and have continuing effects, annual emissions would have to be cut by at least 80% to get our production of greenhouse gases back in line with what the Earth's systems can deal with. Only by keeping emissions at the level the planet can absorb can we stabilize the atmospheric concentration of CO2.

Note: this is only talking about stabilizing the atmospheric concentration of carbon dioxide at some higher level in the future. There is no way to turn back the emissions clock to get to lower levels of CO2 that would commit us to less warming. (The report explicitly didn't consider "geoengineering" to get carbon out of the atmosphere.)

So far, some rich nations have collectively proposed to cut emissions by about 12.5% below 1990 levels by 2020 (see this previous post). Other countries, however, plan to keep increasing emissions. And the promised cuts are not firm. Some have not even been officially embodied in law or policy. There is still vague talk about cutting emissions 50% below 1990 levels by 2050. But even that would result in more CO2 accumulating in the atmosphere every year. Our 1990 emissions, about 20 gigatonnes of carbon dioxide, were already far more than natural systems could absorb, and even half that level would exceed the capacity of natural sinks and result in continued accumulation.

So 50% cuts from 1990 global carbon dioxide emissions would not make atmospheric CO2 concentrations level off. This report considers what warming would result when and if we eventually get CO2 in the atmosphere to stabilize at some future higher level. If we don't achieve such stabilization, warming will continue beyond the temperatures discussed in this study.

We probably can't go back again to earlier historical CO2 levels. The only question is what level we can get those CO2 levels to stabilize at, if any. That will determine what degree of global warming we will eventually have.

Our legacy

If you think the heat waves and other very minor consequences we have seen with just one degree of warming are a cause for concern, you ain't seen nothing yet. We are already committing our great grandchildren to several degrees of additional warming--maybe three or four more degrees--a very different Earth. The question is how much we further we are willing to go beyond that.

[All quotations and images are from "Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia" by the Committee on Stabilization Targets for Atmospheric Greenhouse Gas Concentrations of the Board on Atmospheric Sciences and Climate Division on Earth and Life Studies, National Research Council of the National Academies. Published by The National Academies Press, prepublication copy, at http://www.nap.edu/catalog/12877.html. The report is copyright © National Academies of Sciences.]

13 July 2010

The Temperature's Rising--It Isn't Surprising

Weather and Climate--Uncomfortable Truths

Recent heat waves in Eastern North America and Eastern Asia call attention to some sticky trends. While it's important to distinguish between "weather", what you see outside at a particular place and time, and "climate", long-term average weather, a number of scientific studies say that future decades will probably see many more heat waves like these.

Sections below cover: Predictions from models, recent weather records, health effects, and other effects of hot weather.

Previous "record" heatwaves will become commonplace

Noah Diffenbaugh and Moetasim Ashfaq at Stanford report in "Intensification of hot extremes in the United States", in press at Geophysical Research Letters (GRL), that what was a record heat wave during the 20th century could occur several times a decade over the next 30 years.
In the study, Diffenbaugh and Ashfaq used two-dozen climate models to project what could happen in the U.S. if increased carbon dioxide emissions raised the Earth's temperature by 1.8 degrees Fahrenheit (1 degree Celsius) between 2010 and 2039 ... . ... "Our results suggest that limiting global warming to 2 degrees C above preindustrial conditions may not be sufficient to avoid serious increases in severely hot conditions," Diffenbaugh said. ... According to the climate models, an intense heat wave - equal to the longest on record from 1951 to 1999 - is likely to occur as many as five times between 2020 and 2029 over areas of the western and central U.S. The 2030s are projected to be even hotter. "Occurrence of the longest historical heat wave further intensifies in the 2030-2039 period, including greater than five occurrences per decade over much of the western U.S. and greater than three exceedences per decade over much of the eastern U.S.," the authors wrote. [Quoting from Woods Institute post.]
Image from Diffenbaugh and Ashfaq via Woods Institute post

A post at The Woods Institute for the Environment at Stanford University summarizes their recent research. The paper hasn't appeared yet at GRL (and will probably be behind a pay wall there when it is published).

Other similar findings

The 2009 report of the U.S. Global Change Research Program, Global Climate Change Impacts in the U.S., found "Many types of extreme weather events, such as heat waves and regional droughts, have become more frequent and intense during the past 40 to 50 years." "Recent studies using an ensemble of models show that events that now occur once every 20 years are projected to occur about every other year in much of the country by the end of this century. In
addition to occurring more frequently, at the end of this century these very hot days are projected to be about 10°F hotter than they are today."

Hayhoe et al. report in Climate change, heat waves, and mortality projections for Chicago (in press) that events like the 1995 heat wave that tortured Chicago "could occur every other year on average under lower emissions and as frequently as three times per year under higher." Considering an event like the European Heat Wave of 2003, "Between mid- and end-of-century, there could be as many as five such events under lower, and twenty-five under higher emissions." The abstract:
Over the coming century, climate change is projected to increase both mean and extreme temperatures as heat waves become more frequent, intense, and long-lived. The city of Chicago has already experienced a number of severe heat waves, with a 1995 event estimated to be responsible for nearly 800 deaths. Here, future projections under SRES higher (A1FI) and lower (B1) emission scenarios are used to estimate the frequency of 1995-like heat wave events in terms of both meteorological characteristics and impacts on heat-related mortality. Before end of century, 1995-like heat waves could occur every other year on average under lower emissions and as frequently as three times per year under higher. Annual average mortality rates are projected to equal those of 1995 under lower emissions and reach twice 1995 levels under higher. An 'analog city' analysis, transposing the weather conditions from the European Heat Wave of 2003 (responsible for 70,000 deaths across Europe) to the city of Chicago, estimates that if a similar heat wave were to occur over Chicago, more than ten times the annual average number of heat-related deaths could occur in just a few weeks. Climate projections indicate that an EHW-type heat wave could occur in Chicago by mid-century. Between mid- and end-of-century, there could be as many as five such events under lower, and twenty-five under higher emissions. These results highlight the importance of both preventive mitigation and responsive adaptation strategies in reducing the vulnerability of Chicago's population to climate change-induced increases in extreme heat.

If you think this year has been hotter than you remember, NASA agrees

We have just experienced the hottest January to June in the NASA dataset, which goes back to 1880. "It is likely that global temperature for calendar year 2010 will exceed the 2005 record, but that is not certain if a deep La Nina develops quickly." (From the June 1, 2010, Revised draft of Dr. James Hansen's "Global Surface Temperature Change" paper.)

12-month mean global temperature is now the warmest
in instrumental record. 60 and 132-month (5 and 11-year)
means minimize tropical and solar variability. These charts
and the caption are from Hansen's poster, available here (ppt).

(Joe Romm's Climate Progress blog summarizes the data from various sources in this post.)

More than inconvenient: People die in heat waves, but you'll probably survive

Most discussions of heat waves point out that they are the deadliest weather events, with more people dying from heatwaves than from floods, cold or hurricanes. That is true. But most of the people that die in heat waves are the elderly and those with serious medical conditions. Other weather events kill more randomly.

Mortality may increase during heat waves, but the drop in mortality that may occur after heat waves suggest that some of those deaths may be "short-term forward mortality displacement". Some studies have shown that a large fraction of heat wave deaths may have been among people who would have died within a short time even without the extreme hot weather. (For example see Revich and Shaposhnikov, 2008.

Deaths in floods and hurricanes are not concentrated among the elderly who may have been on the point of death, but have broader age distributions. The elderly and ill do suffer more in cold weather, but there isn't the same evidence for forward mortality displacement.

More than just hot weather

Yes it will be hotter, and heat waves will be more frequent and more severe. But many other effects are correlated with temperature.
  • Insurance claims for lightning damage are strongly correlated with temperature. These will probably increase. Premiums will go up.
  • Crop damage from heat waves can be significant, especially if they happen during sensitive times like germination, flowering or pollination.
  • The incidence of several diseases, including Salmonella food poisoning and West Nile Virus, are correlated with high temperature events.
  • Heat waves can lead to crime waves, especially if they cause power blackouts.

Get used to a very different Earth

"We're getting a dramatic taste of the kind of weather we are on course to bequeath to our grandchildren," says Tom Peterson, Chief Scientist for NOAA’s National Climatic Data Center.  (Quoted on The Project on Climate Science site.)

Thanks to Irving Berlin for the lyric of the song "Heat Wave" used for the title of this post. Hear here